In the process of exploring for hydrocarbon resources, seismic prospecting produces considerable data in the form of squiggles and echoes. Processing and interpreting the data to develop an understanding of the shape and contours of the earth formations requires considerable skill and experience, but still produces considerable ambiguity and uncertainty.
One area of uncertainty and ambiguity is at the bottom portions of high velocity layers such as salt, basalt, etc. where lower velocity sediments underlie the high velocity layers. Salt domes, for example, tend to transmit seismic energy very fast. Other materials typically transmit seismic energy comparatively slower. It is also generally believed, especially as it relates to the Gulf of Mexico, that there are many large, high velocity formations overlying significant volumes of hydrocarbon bearing sediments. Unfortunately, the high velocity layers create significant challenges for geoscientists to confidently resolve prospects for hydrocarbon production. The challenge arises mainly because seismic energy is refracted as it crosses ordinary geological interfaces according to Snell's law. However, when seismic energy crosses interfaces with high velocity materials on one side and much lower velocity materials (comparatively) on the other, the refraction is greatly exaggerated. With exaggerated refraction occurring at these boundaries or interfaces, substantial dip angles essentially prevent useful seismic energy from being acquired and/or processed in a conventional manner. Essentially, the orientation of the high/low velocity interface and the velocity contrast there as they relate to the path of the seismic energy can easily deflect the seismic energy such that very little energy arrives at any location above that interface for detection.
It should also be understood that the interfaces beneath the bottom of the salt tend to be highly irregular and convoluted. With substantial complexity and exaggerated affect, salt domes and other high velocity geologies are quite problematic for seismic prospecting. The seismic data records for such areas often show no data, and therefore no interface imaged by them between highly distinct geological layers. The absence of such data is particularly perplexing if there are indications that hydrocarbons could potentially be pooling in one or more locations somewhere along a very broad area in one or more geological formations but hidden below the high velocity layer.
In the past, those involved with hydrocarbon exploration simply avoided subsalt formations. While it is believed that considerable hydrocarbon resources may underlie many salt domes in the Gulf of Mexico, the cost of drilling wells offshore, and especially in deep water, is far too expensive to take such blind risks. Considerable information is desired and analysis is performed to reduce the risk of drilling very expensive dry holes. Even with very good information, more dry holes are drilled than profitable wells so the expense and effort to reduce risk is clearly justified.
As more seismic data is collected in the Gulf of Mexico, the size and locations of salt domes are known and more can be gleaned as to the shapes of the perimeter and near perimeter bottoms of the salt domes. Information that reveals the shape of the bottom of the salt domes make it possible to understand what is happening to the seismic energy transiting through the salt domes and other high velocity layers and may allow better understanding of the formations under such salt domes. With better understanding of the shapes of salt domes, it becomes more practical to investigate promising subsalt prospects with additional seismic or other investigative techniques. However, additional seismic and/or re-processing of existing data is expensive, and can end up not providing any helpful information. Knowing what information is available from each of various techniques, a simple cost benefit analysis may be performed to determine whether the additional effort will provide information that will lead to better drill or not to drill or where to drill decisions.